A wide assortment of endoluminal prostheses have been developed, each providing a uniquely beneficial structure to modify the mechanics of a targeted lumen wall within a body lumen. As used herein, “endoluminal prosthesis” is intended to cover a medical device that is adapted for temporary or permanent implantation within a body lumen, including both naturally occurring and artificially made lumens. For example, stent prostheses are known for implantation within body lumens to provide artificial radial support to the wall tissue, which forms the various lumens within the body, and often more specifically, for implantation within the blood vessels of the body. A stent may provide long-term support for damaged or traumatized wall tissues of the lumen or may be implanted, for example, to maintain the patency restored to a blood vessel that was clogged with atherosclerotic plaque. There are numerous conventional applications for stents including cardiovascular, urological, gastrointestinal, and gynecological applications.
Essentially, stents are made to be permanently or temporarily implanted. A permanent stent is designed to be maintained in a body lumen for an indeterminate amount of time and is typically designed to provide long-term support for damaged or traumatized wall tissues of the lumen or to maintain the patency of a vessel clogged with atherosclerotic plaque. A temporary stent is designed to be maintained in a body lumen for a limited period of time in order to maintain the patency of the body lumen, for example, after trauma to a lumen caused by a surgical procedure or an injury or to temporarily open a clogged lumen until natural healing occurs.
Permanent stents, over time, may cause irritation to the surrounding tissue resulting in inflammation at the implant site and restenosis, or re-narrowing of the vessel lumen. Further, if an additional interventional procedure is ever warranted, a previously permanently implanted stent may make it more difficult to perform the subsequent procedure.
Temporary stents, on the other hand, avoid the complications associated with long-term implants. Temporary stents may advantageously be eliminated from body lumens after an appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the patency of the lumen. Temporary stents may be made from bioabsorbable and/or biodegradable materials that are selected to absorb or degrade in vivo over time. Materials, typically bioabsorbable polymers, and processes typically used to produce bioabsorbable stents result in stents with low tensile strengths and low modulus, compared to metallic stents of similar dimensions. The limitations in mechanical strength of the bioabsorbable stents can result in stent recoil after the stent has been inserted. This can lead to a reduction in luminal area and hence blood flow. In severe cases the vessel may completely re-occlude. In order to prevent the recoil, polymeric stents have been designed with thicker struts (which lead to higher profiles) or as composites to improve mechanical properties. The use of relatively thick struts makes polymeric stents stiffer and decreases their tendency to recoil, but a significant portion of the lumen of the artery can be occupied by the stent. This makes stent delivery more difficult and can cause a reduction in the area of flow through the lumen. A larger strut area also increases the level of injury to the vessel wall and this may lead to higher rates of restenosis i.e. re-occlusion of the vessel. Thus, current totally bioabsorbable stents lack the mechanical properties to sustain vessel lumen size. Even by design manipulation of past bioabsorbable braid configurations and a selection of the best possible known bioabsorbable materials, the totally bioabsorbable stent falls short of current clinically proven products (i.e., metal stents).
It is therefore an object hereof to provide a hybrid stent which has mechanical properties desired in a stent, such as hoop or radial strength, while maintaining at least a mostly bioabsorbable structure. Such a stent provides sufficient support in a body lumen for the duration of a therapeutically appropriate period of time, after which the biodegradable portion then degrades to be eliminated from the patient's body without surgical intervention.